A directional rotational relaxation model for nitrogen using molecular dynamics simulation

We use Molecular Dynamics (MD) simulation to investigate rotational relaxation in nitrogen from a first-principles perspective. The rotational relaxation process is found to be dependent not only on the near-equilibrium temperature, but more importantly on both the magnitude and direction of the initial deviation from the equilibrium state. Although this dependence has been previously recognized, it is here investigated systematically. The comparison between MD and Direct Simulation Monte Carlo (DSMC), based on the Larsen-Borgnakke model, for shock waves (both at low and high temperatures) and onedimensional expansions shows that a judicious choice of a constant Z_rot can produce DSMC results which are in relatively good agreement with MD. However, the selection of the rotational collision number is case-specific, depending not only on the temperature range, but more importantly on the type of flow (compression or expansion). Parker's model, with the commonly used parameters for nitrogen suggested by Lordi and Mates, overpredicts the magnitude of Z_rot for temperatures above about 300 K. Finally, based on the MD data, a preliminary formulation for a novel directional rotational relaxation model, which includes a dependence on both the rotational and the translational state of the gas, is presented.